How do I render a sector (pie/cake slice) of a circular Unity 2D GameObject?

If writing a proper HLSL shader seems intimidating, you can always just make a Shader Graph. Here's one I designed that you can copy once you have Shader Graph installed:

The variables (top left) are:

• T: The 0-1 variable that determines what percentage of the sprite is visible.
• Start Angle: The angle above the horizontal where the first sliver of the sprite will appear when you make T > 0, expressed in degrees.
• Clockwise: Determines whether the visible sliver of the sprite will expand clockwise or counterclockwise as T increases. Make sure this is a boolean keyword, or else one of the nodes I've used won't be available.
• MainTex: As long as you name the variable that, Unity will automatically set it to whatever's in the SpriteRenderer's "Sprite" field.

If you don't see the Alpha and Alpha Clip Threshold nodes in the "Fragment" block, go to the Graph Settings and turn on "Alpha Clipping".

Next, create a Material that uses this Shader Graph, which you can do by right-clicking the graph in the Project View and selecting Create -> Material.

Go to the SpriteRenderer on the object you want to apply this effect to, and set the "Material" field to this new Material.

Next, add this script to the object:

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

[RequireComponent(typeof(SpriteRenderer))]
public class RadialWipe : MonoBehaviour
{
    [Range(0, 1)] public float t;
    new SpriteRenderer renderer;

    void Awake()
    {
        renderer = GetComponent<SpriteRenderer>();
    }

    void Update()
    {
        renderer.material.SetFloat("_T", t);
    }
}

Now, if you press play and change t in this script's inspector, you'll see the sprite "filling up" like you wanted. You can even have multiple objects like this, all with different t values. Hope this helped!